1. Field of the Invention
The present invention relates to radiation-curable powder coating compositions based on crystalline urethane acrylates, which crosslink to provide light-stable and weather-stable coating films.
2. Description of the Background
Thermal crosslinkable powder coating materials are known and are frequently used in the coatings-processing industry. For example, DE-C 27 35 497 describes PU powder coatings having outstanding weathering stability and thermal stability. The crosslinkers whose preparation is described in DE-C 27 12 931 are composed of ε-caprolactam-blocked isophorone diisocyanate containing isocyanurate groups. Also known are polyisocyanates containing urethane, biuret or urea groups, whose isocyanate groups are likewise blocked.
The disadvantage of these systems lies in the elimination of the blocking agent in the course of the thermal crosslinking reaction. Since the blocking agent may therefore be emitted into the environment, it is necessary on ecological and workplace safety grounds to take special precautions to clean the outgoing air and/or to recover the blocking agent. Moreover, the reactivity of the crosslinkers is low. Curing temperatures above 170° C. are required.
Both disadvantages, emission of blocking agent and curing at high temperatures, can be eliminated by means of powder coating materials which cure, not thermally, but instead by means of radiation. Radiation-curable powder coating materials of this kind are known and are described, for example, in U.S. Pat. No. 3,485,732, EP 0407826, EP 0636669, WO 99/14254, U.S. Pat. Nos. 3,974,303, 5,639,560 and EP 0934359.
EP 0636669 and WO 99/14254 describe two-component radiation-curable powder coating materials based on an unsaturated polyester and on a vinyl ether. The coatings produced from them are unsuitable for exterior use, since they yellow severely.
U.S. Pat. No. 3,974,303 describes thermoplastic resins containing from 0.5 to 3.5 polymerizable unsaturated groups per 1000 g molecular weight. The polyurethanes described (see Example 1) are, however, not weather-stable and, on account of the absence of polyester groups and the low chain length, possess a low flexibility.
EP 0934359 describes pulverulent, radiation-curable mixtures of amorphous and crystalline polyesters containing terminal methacrylate groups. The flexibility and adhesion of the powder coatings produced from these mixtures are no more than satisfactory.
U.S. Pat. No. 5,639,560 describes radiation-curable powder compositions comprising special crystalline polyesters, containing methacrylate groups terminally, as binders. The binders are prepared by reacting crystalline polyesters with glycidyl methacrylate. The coatings produced from these powder coating compositions are very flexible.
There remains a continuing need for radiation-curable powder coating compositions which overcome the difficulties described above.